FIG. 13 shows one example structure from the prior art for detecting the bending angle 2 .theta. of a measurement object comprised of a workpiece W which has undergone a bending process in cooperation with a punch P and a die D equipped in a press brake (omitted from the drawings), wherein an upward bend angle 2 .theta. is determined by raising or lowering a detection element 101 to bring the tip of the detection element 101 into contact with a lower surface of the workpiece W, with this value then being doubled to determine the bending angle 2 .theta..
In the above-described structure of an angle detection apparatus 103, an ascension/descension device 107 such as a cylinder is provided on a die base 105 equipped with a die D, and by means of this ascension/descension device 107 the detection element 101 is raised in an upward direction in FIG. 13 to be brought into contact with a lower surface of the workpiece W. The raised position of the detection element at this time can be measured, for example, by a pulse encoder 113 connected to a pinion gear 111 which meshes with a rack 109 arranged to move up and down together with the detection element 101, whereby a structure is formed for detecting the bending angle 2 .theta..
Further, a process for detecting the bending angle of the workpiece may be carried out by imaging an edge surface of the bent up workpiece with a non-contact imaging means and then processing such imaged data with a image processing.
Furthermore, there is another angle measurement apparatus, in which a rotatable frame equipped with a light source and a converging lens is provided, and in which a plurality of light-receiving elements are arranged in the shape of a circular arc at a location corresponding to the focus position of the converging lens, whereby reflected light of the light which emitted toward the workpiece from the light source is focused by the converging lens toward the position of the light-receiving elements arranged in the shape of the circular arc, whereby the position of the light-receiving elements where the received light is a maximum is detected in order to detect the bending angle of the workpiece.
However, there is a problem with regards to these types of prior art technology in that highly accurate angle detection is difficult because the bending angle is determined from the relationship between the position of the die D and the position of the workpiece W determined by measuring the position of the detection element 101 in contact with the workpiece W.
Further, because angle detection is carried out by bringing the detection element 101 into contact with the workpiece W, deformation of the workpiece W will arise depending on the strength of contact with the detection element, and this risk of changes arising in the bending angle creates a problem.
Further, in the case where an image processing device is used, because the apparatus is a non-contact type, there is no risk of changes arising due to contact, but high costs and the ability to only measure an edge surface of the workpiece creates problems.
In the structure having a plurality of light-receiving elements arranged in a circular arc shape at positions corresponding to the focus of the rotating converging lens, because accurate measurements are difficult if the light-receiving elements are shifted even a small amount from the focus position of the converging lens, there is a need for high precision in the apparatus and this creates problems.
In view of the problems of the prior art described above, it is an object of the present invention to provide an angle detection method for bending machines, an angle detection apparatus and an angle sensor which make it possible to carry out highly accurate angle detection by detecting the angle of a bent workpiece without making contact with the surface of the workpiece.